1. Field of the Invention
The present invention relates to a material for forming a passivation film for a semiconductor substrate, a passivation film for a semiconductor substrate and a method of producing the same, and a photovoltaic cell element and a method of producing the same.
2. Background of the Invention
Conventional methods of producing silicon photovoltaic cell element will be explained.
First, a p-type silicon substrate, on which a texture is formed in order to improve efficiency by promoting a light-trapping effect, is prepared. Subsequently, a n-type diffusion layer is uniformly formed by carrying out treatment in an atmosphere of mixed gas of phosphorus oxychloride (POCl3), nitrogen and oxygen, at from 800° C. to 900° C. for several ten minutes. According to the conventional method such as this, since diffusion of phosphorus is conducted by using a mixed gas, n-type diffusion layer are formed not only on a front surface but also on side and back surfaces. Therefore, side etching is carried out in order to remove the n-type diffusion layers formed on the side surfaces. Further, the n-diffusion layer formed on the back surface needs to be converted to a p+-type diffusion layer. Therefore, ohmic contacts are obtained by printing an aluminum paste on the back surface and sintering the same, at the time of converting the n-type diffusion layer to a p+-type diffusion layer.
However, an aluminum layer formed from an aluminum paste exhibits low conductivity. Therefore, in order to lower the sheet resistance, the aluminum layer formed on the entire back surface typically needs to have a thickness after sintering of about from 10 μm to 20 μm. Further, the coefficients of thermal expansion of silicon and aluminum are significantly different. Therefore, a large internal stress is created in a silicon substrate during sintering and cooling, and the internal stress may damage crystal grain boundaries, increase crystal defects or cause warpage.
In order to solve the problems as described above, there is a method of reducing the thickness of back-surface electrode layers by reducing the amount of an aluminum paste to be applied. However, if the amount of the aluminum paste is reduced, the amount of aluminum that diffuses into the inside of a p-type silicon semiconductor from its surface may be insufficient. As a result, a desired BSF (Back Surface Field) effect, i.e., an effect of improving collection efficiency of photogenerated carriers due to the presence of the p+-type diffusion layer, may not be attained and the performances of the photovoltaic cell may be lowered.
In connection to the above, a technique of partially forming p+ layers and aluminum electrodes by applying an aluminum paste onto portions of a surface of a silicon substrate, referred to as point contacts, has been proposed (see, for example, Japanese Patent No. 3107287).
In a case of photovoltaic cells having a point-contact structure on a surface opposite to a light-receiving surface (hereinafter, also referred to as a back surface), the rate of recombination of minority carriers needs to be suppressed at surfaces corresponding to portions other than aluminum electrodes. In order to achieve this goal, films of SiO2 and the like have been proposed as a passivation film for the back surface side (see, for example, Japanese Patent Application Laid-Open No. 2004-6565). The passivation film reduces the surface state density, which causes recombination, by terminating dangling bonds of silicon atoms at a surface portion of the back surface of the silicon substrate, by forming an oxide film on the back surface of the silicon substrate.
Further, a method of utilizing a film of SiNx (silicon nitride), which is widely used as an antireflection film for the light-receiving surface side, also as a passivation film for the back surface has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2010-537423).
However, the SiO2 film and the SiNx film proposed in Patent Document 2 and Patent Document 3 are typically formed via a thermal oxidation method or a CVD method. In a thermal oxidation method, performing a high-temperature treatment at 1000° C. or higher is generally necessary, and process conditions, such as gas a flow rate or a gas flow rate distribution, need to be controlled.
In addition, when a CVD apparatus is used, there is a case in which an effect of hydrogen passivation caused by decomposition of a reactant gas may be expected depending on the type of the reactive gas. However, there are problems in that the throughput is low and that the production costs are high due to the need for frequent maintenances. In addition, since formation of openings in a passivation film for a back side is typically carried out by photolithography, there are problems in terms of the number of processes, production costs and the like.